This invention relates to ceramic dielectric compositions which have high dielectric constants (K), e.g., between about 3000 and about 4700; low dissipation factors (DF), e.g., below about 2.5%; high insulation resistance (R) capacitance (C) products (RC), e.g. above about 5,000 ohm-farad at 25.degree. C., above about 1000 ohm-farad at 125.degree. C.; and stable temperature coefficient (TC) in which the dielectric constant does not alter from its base value at 25.degree. C. by more than about 15 percent over a temperature range from -55.degree. C. to 125.degree. C.
Multilayer ceramic capacitors (MLCs) are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder; placing thereupon conducting metal electrode layers, usually a palladium/silver alloy in the form of metallic paste; stacking the resulting elements to form the multilayer capacitor; and firing to densify the material thus forming a multilayer ceramic capacitor. Other processes for forming MLCs are described in U.S. Pat. Nos. 3,697,950 and 3,879,645 the texts of which are incorporated herein by reference.
Barium titanate (BaTiO.sub.3) is one of the major components most frequently used in the formation of the ceramic dielectric layers because of its high dielectric constant. However, the variation of the dielectric constant with temperature and the insulation resistance are also important factors to be considered in preparing ceramic compositions for use in multilayer capacitors. The electrical properties of many dielectric ceramic compositions may vary substantially as the temperature increases or decreases. Other factors also affect the electrical properties of ceramic compositions, e.g., insulation resistance may vary substantially based on grain sizes after final sintering.
In a desirable dielectric ceramic composition for use in a multilayer capacitor for applications requiring stability in the dielectric constant over a wide temperature range, the dielectric constant does not change from its base value at 25.degree. C. (room temperature) by more than about plus or minus 15 percent. The insulation resistance and capacitance product of such compositions should be more than 1000 ohm-farads at 25.degree. C. and more than 100 ohm-farads at maximum working temperature, 125.degree. C. in most cases. The method commonly used to produce such temperature stable capacitors consists of firing BaTiO.sub.3 together with minor oxide additives for controlling the final dielectric properties. However, the dielectric ceramic compositions known in the art for making multilayer capacitors having stable TC characteristics usually have dielectric constants of not more than about 3000.
Because of their high dielectric constants, low dissipation factors, and stable TC characteristics, the ceramic compositions of this invention provide advantages in manufacturing multilayer ceramic capacitors, hereinafter called MLC's, such as high capacitance and small physical size. These advantages are very important for the capacitor manufacturing companies to meet the ever increasing demands of technology advance and cost reduction.